The DAK and LEM are both attempts to mirror some aspects of a striker-fired system like the Glock with a hammer-fired pin - i.e. a consistent and lighter trigger pull. I've shot the Sig DA/SA extensively, thought I liked the DAK but now realize its limitations and am trading it in for a LEM. I've only shot a few boxes through XD's and Glocks, enough to know I can't stand the creepy pull.
DAK - a DAO with a long smooth 6.5# trigger pull. Resets at about half length for a 8.5# pull (accidental byproduct of the second-strike capability), and at full length for the default 6.5# pull. It uses leverage to produce the lighter pull, so the length of the pull is a necessary consequence of lightening the trigger. It is very similar to a revolver-style DAO pull and revolver shooters seem to like it a lot. The long pull breaks very cleanly, so you're less likely to anticipate the shot. But the long reset limits fast shooting. I bought the DAK thinking I'd improve my controlled pairs due to the consistent trigger pull, but I actually find them easier with a DA/SA. I can train to the DA/SA but I can't train around a long reset - I can't push the trigger back forward any faster. Todd Green and several other instructors involved with the DAK recommend not using the short reset at all and I believe that's the way they teach it at Sig Academy.
LEM - more akin to a single-action or striker fired system. There's an internal portion of the hammer that is cocked by the action of the slide. The trigger merely cocks the hammer the rest of the way. The first pull is long and light, but the main difference is that the reset is short (about 1/3rd travel) but has the same weight as the first pull. Depending on the version, this is anywhere from 4-7 lbs. Moreover, there is the option of a heavier reset spring to more rapidly reset the trigger and allow for a followup shot.
I've not yet shot the LEM but at least on paper it looks like a superior solution and I'm looking forward to giving it a try - if I can find a P30 LEM for a decent price.